[GlobalISel] LegalizerHelper: Implement fewerElementsVector for G_LOAD/G_STORE

Reviewers: aemerson, dsanders, bogner, paquette, aditya_nandakumar

Reviewed By: dsanders

Subscribers: rovka, kristof.beyls, javed.absar, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D53728

llvm-svn: 349200